Multistage residual oil hydrodesulfurization process employing segmented feed addition and product removal

Abstract

In the catalytic hydrodesulfurization of residual oil the amount of hydrogen consumed per atom of sulfur removed is relatively low until the desulfurization becomes deep, whereupon the amount of hydrogen consumed per atom of sulfur removed becomes relatively high. The present invention provides a multistage process capable of producing products of low sulfur content while avoiding deep desulfurization of the heavy portion of the residual oil so that hydrogen consumption is diminished. The feed oil is fractionated to provide a residual fraction, a heavy distillate fraction and a light distillate fraction. The residual fraction and hydrogen are charged to an upstream hydrodesulfurization stage. A portion of the upstream stage residual oil effluent stream is split out of the process for use as refinery fuel and the remaining portion of the upstream stage effluent stream is charged to an intermediate hydrodesulfurization stage together with the heavy distillate feed fraction and hydrogen. A portion of the intermediate stage effluent stream is split out of the process as product fuel oil and the remaining portion of the intermediate stage effluent stream is passed to a downstream hydrodesulfurization stage together with the light distillate feed fraction and hydrogen. The downstream stage effluent stream constitutes the final and highest grade product of the process. Because of the combination of segmented feed addition and segmented product removal, not only are residue-containing streams removed from the process at the earliest possible time to avoid overtreating relative to their intended use, but also the removed streams are diluted with a reduced amount of low boiling distillate oil, conserving as much of the low boiling distillate oil as possible for inclusion in the product of the final stage. This method conserves distillate feed oil for the final and highest grade product and allows each hydrodesulfurization stage to be provided with a non-aliquot distillate-residual oil stream which is richer in low boiling distillate oil than its predecessor hydrodesulfurization stage.

We claim: 
     
       1. A process for the hydrodesulfurization of an aromatics- and asphaltene-containing feed oil to produce at least three hydrodesulfurized residual oil streams having different respective asphaltene and sulfur contents, said process employing upstream, intermediate and downstream hydrodesulfurization stages containing hydrodesulfurization catalyst comprising Group VI and Group VIII metal on a noncracking support at a temperature between 600° and 900° F. and a hydrogen pressure between 500 and 5,000 psi, said process comprising fractionating said feed oil into an asphaltene-containing residual oil stream, a heavy distillate oil stream and a light distillate oil stream, passing said residual oil stream and hydrogen through said upstream hydrodesulfurization stage and recovering an upstream stage effluent stream containing refractory sulfur asphaltenes, splitting said upstream stage effluent stream into a first upstream stage effluent portion comprising between about 10 and 75 weight percent of the normally liquid material in said upstream stage effluent stream and a second upstream stage effluent portion, removing said first upstream stage effluent portion from said process to selectively remove refractory sulfur asphaltenes from said process and to increase the concentration of more sulfur-reactive material in said intermediate stage, passing said second upstream stage effluent portion and said heavy distillate oil stream and hydrogen through said intermediate stage, said heavy distillate oil stream by-passing said upstream stage, recovering an intermediate stage effluent stream containing refractory sulfur asphaltenes, dividing said intermediate stage effluent stream into a first intermediate stage effluent portion comprising between about 10 and 75 weight percent of the normally liquid material in said intermediate stage effluent stream and a second intermediate stage effluent portion, removing said first intermediate stage effluent portion from said process to selectively remove refractory sulfur asphaltenes from said process and to increase the concentration of more sulfur-reactive material in said downstream stage, passing said second intermediate stage effluent portion and said light distillate oil stream and hydrogen through said downstream stage, said light distillate oil stream by-passing said upstream and intermediate stages, and recovering a downstream stage effluent stream, said downstream stage effluent stream containing aromatics and having a sulfur concentration which is at least 75 percent lower than the sulfur concentration of said feed oil, the removal of said first upstream stage effluent portion and said first intermediate stage effluent portion allowing the sulfur concentration in said downstream stage effluent stream to be achieved with a relatively high aromatics concentration as compared to the aromatics concentration when achieving the same sulfur concentration by hydrodesulfurization without selective removal of refractory sulfur asphaltenes. 
     
     
       2. The process of claim 1 wherein said first upstream stage effluent portion comprises between about 30 and 50 weight percent of the normally liquid material in said upstream stage effluent stream. 
     
     
       3. The process of claim 1 wherein said first upstream stage effluent portion comprises between about 20 and 65 weight percent of the normally liquid material in said upstream stage effluent stream. 
     
     
       4. The process of claim 1 wherein said first intermediate stage effluent portion comprises between about 30 and 50 weight percent of the normally liquid material in said intermediate stage effluent stream. 
     
     
       5. The process of claim 1 wherein said first intermediate stage effluent portion comprises between about 20 and 65 weight percent of the normally liquid material in said intermediate stage effluent stream. 
     
     
       6. The process of claim 1 wherein no more than 30 percent of said feed oil boiling above 650° F. is converted to material boiling below 650° F. 
     
     
       7. The process of claim 1 wherein not more than 10 percent of said feed oil boiling above 650° F. is converted to material boiling below 650° F. 
     
     
       8. The process of claim 1 wherein the catalyst in an intermediate or downstream hydrodesulfurization stage contains a promoting amount of Group IV-B metal.